What do you feed into inputs A/B

Hi, I have been using my machinedrum mk2 (not UW) a long time since I received it. I paired a DSI evolver as a melodic instrument, I really like the pairing and so I’m thinking about what other synths it might be fun to pair with. Having sold my A4 today I have some bucks to spend and was thinking of two mono to cover a bass line and one to do something else, advice? opinions? very tempted by a TD-3 and microfreak (cheap and flexible).